C# code to switch off certificate validation

smtp.cs

{
  ...
  ServicePointManager.ServerCertificateValidationCallback =
      delegate(object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) {
          return true;
      };

  smtpclient.Send();
  ...
}

// OR


{
  ...
  ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CertificateValidationCallBack);

  smtpclient.Send();
  ...
}

private static bool CertificateValidationCallBack(
   object sender,
   System.Security.Cryptography.X509Certificates.X509Certificate certificate,
   System.Security.Cryptography.X509Certificates.X509Chain chain,
   System.Net.Security.SslPolicyErrors sslPolicyErrors) {
  // If the certificate is a valid, signed certificate, return true.
  if (sslPolicyErrors == System.Net.Security.SslPolicyErrors.None) {
    return true;
  }

  // If there are errors in the certificate chain, look at each error to determine the cause.
  if ((sslPolicyErrors & System.Net.Security.SslPolicyErrors.RemoteCertificateChainErrors) != 0) {
    if (chain != null && chain.ChainStatus != null) {
      foreach (System.Security.Cryptography.X509Certificates.X509ChainStatus status in chain.ChainStatus) {
        if (
          (certificate.Subject == certificate.Issuer) &&
          (status.Status == System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.UntrustedRoot)) {
          // Self-signed certificates with an untrusted root are valid. 
          continue;
        }
        else {
          if (status.Status != System.Security.Cryptography.X509Certificates.X509ChainStatusFlags.NoError) {
            // If there are any other errors in the certificate chain, the certificate is invalid,
            // so the method returns false.
            return false;
          }
        }
      }
    }
    
    // When processing reaches this line, the only errors in the certificate chain are 
    // untrusted root errors for self-signed certificates. These certificates are valid
    // for default Exchange server installations, so return true.
    return true;
  }
  else {
    // In all other cases, return false.
    return false;
  }
}